In some taps of the quarter turn type referred to above, an arrangement of discs, usually of ceramics, incorporating apertures therethrough has been proposed. Adjacent faces of the discs are mounted to abut each other such that rotation of one of the discs relative to the other brings the apertures through the respective discs into and out of alignment to allow operation of the tap between fully opened and fully closed conditions with only a part turn of the tap spindle, namely through 90.degree. (quarter turn). A tap of the above general type is disclosed in the specifications of U.S. Pat. No. 4,738,277 and U.K. Pat. No. 2,159,925.
As discussed in the specifications of the aforementioned patent applications, in order to meet the standards set down by water supply authorities, including Australian water supply authorities, and with which taps must comply, the distance between the outer surface surrounding the access opening to the interior of a tap body within which a tap top assembly is to be inserted, to the valve seat within the body, has a value within specified tolerances, and thus, as the inner end of the assembly is required to permanently seat in sealing engagement with the valve seat within the tap body, the corresponding length of the tap top assembly must comply with the specified distance. However, the allowable tolerances can lead to a situation where the relevant length of the tap top assembly may be less than the specified distance for the tap body thus preventing proper sealing engagement with the valve seat in the tap body, or if longer, preventing proper sealing of the tap top assembly with the tap body around the access opening within which the assembly is received.
It is also desirable that tap top assemblies be provided as replacements for tap top assemblies in existing tap bodies so as to enable conversion of an existing tap to a quarter turn tap, and thus the relevant dimensions may not be such as to ensure proper sealing engagement of the tap top assembly in the existing tap body.
In the specifications of both the aforementioned patent applications provision is made to ensure that the inner end of the tap top assembly firmly seats on the valve seat within the tap body and also that proper sealing is achieved between the tap top assembly around the access opening through the valve seat.
Both the assemblies of the specifications of the aforementioned patent applications allow for firm engagement of the inner end of the tap top assembly with the valve seat within the tap body as an initial step in the positioning of the tap top assembly, and as a result have been subject to the problem that during installation, or in subsequent servicing, greater pressure than necessary has been applied to the sealing washer on the end of the tap top assembly such that the washer has been compressed to the extent that the pressure has been transmitted to the disc arrangement against which the washer bears, and which may result in damage to the ceramic discs and therefore cause leakage through the disc assembly and/or failure of the disc assembly to function properly. In addition, if excessive pressure if applied to the ceramic disc assembly, two other effects can result, namely the lubricating medium between the discs may be forced out thus resulting in high friction and greater wear and/or higher operating torque being necessary through the tap spindle.